In a gas turbine engine, the compressor and turbine are supported on a shaft which extends through the engine housing. This shaft is mounted on bearings at various locations in the engine. A lubricating system supplies these bearings with the desired amounts of oil flow.
Basically, the oil is circulated within the system by a positive displacement pump which is driven by the engine shaft. The pump, therefore, is characterized by a flow rate which varies in direct proportion to engine speed.
The bearing is mounted about the shaft within a housing which is sealed at the shaft. Oil is pumped into the housing, sprayed onto the bearing and collected at the bottom of the housing to be drained into a sump. Depending on the application, drainage can be accomplished in various ways, for example, gravity, additional pumps, or bleeding high pressure air through the shaft seals. Gravity may be used only where there is sufficient room to allow for a large drainage area to insure that all of the oil flow can be drained. However, in general, drainage is impaired by the necessity of using passages having a small cross-sectional area. Therefore in many instances, problems begin to arise as the engine speed increases and the oil flow overtakes the capability of the drainage system.
Drainage may be aided by the use of the high pressure air wich is bled from the compressor stage to pressurize the main bearing seals. This high pressure air causes a flow of air into the housing through the shaft seals, thereby increasing the pressure within the housing and creating a force to improve the flow of draining oil from the housing. This method is effective at high speeds to maintain the desired drainage flow. However, its disadvantage is that under idle or shutdown conditions, the air pressure available is substantially reduced, while the pump is still operating at relatively high flow levels. This causes an undesirable build-up of oil in the bearing package resulting in greater heat absorption in the oil. Because of the low pressure differential across the seals, oil can leak through the main shaft seal and cause oil smoking of the engine.
In order to eliminate this problem, a unique oil supply system is designated to bypass excess oil flow from the pump during idle conditions and to shut off oil flow after shut-down of the engine.